Process for the production of foundry cores and molds



United States Patent 7,211/68 US. Cl. 164-43 9 Claims Int. Cl. B22c 1/22, 9/12, 15/20 ABSTRACT OF THE DISCLOSURE A process for the preparation of foundry cores and molds having the advantage of a rapid cure of the binder employed is achieved by providing a mixture of sand, a curable binder, and a polyisocyanate, and after introducing the such mixture into a core box, mold, or pattern to form a green core or mold passing a volatile amine therethrough.

The present invention relates to the production of cores and molds for use in the production of metal castings, which cores and molds comprise a foundry aggregate, such as sand, which has been formed into a coherent mass with a binder; more particularly, the present invention relates to an improved process of providing a rapid cure of the binder employed by mixing such curable binder with polyisocyanate, and thereafter effecting a rapid cure by the action of a volatile amine.

Cores and molds for use in making metal castings have been normally prepared from mixtures of an aggregate material and a binding amount of a polymerizable or curable binder. The binder allows the mixture to be molded or shaped into the desired form and thereafter cured or allowed to cure to form a self-supporting structure. Minor amounts of modifying materials such as iron oxides, ball clay and other inorganic substances have been included in these mixtures to give a bond at high temperature, i.e., when the binder has been destroyed. Hot strength properties are of importance to the foundry industry because of the requirement of producing dimensionally accurate castings of an acceptable surface finish.

Generally, after the aggregate and the binder have been mixed, the resultant mix is rammed, blown or otherwise introduced into a mold, core box or pattern (hereinafter referred to for convenience as a core box), so as to take the shape of the internal surfaces of such core box. The formed shape then has a given setting or hardening time, i.e., the time which must elapse before sufficient strength has developed in the formed shape to enable it to be removed from the core box. Previously it was the practice to remove the core from the box while still in the green state, i.e., while unhardened, the core later being subjected to a stoving operation which cured the binder and produced a strong, hard shape which could withstand the handling to which it was later subjected. This process suffered from the disadvantage, however, that the cores tended to lose dimensional accuracy due to sagging before and during the stoving cycle.

It has accordingly been realized, therefore, that curing or hardening of the core should take place while the core is still in the core box. There are two main classes of processes for effecting such curing, those carried out in heated boxes and those carried out in cold boxes. The former class, while being capable of producing dimensionally accurate cores at a rapid rate of production has a number of disadvantages, i.e. the cost of heating the boxes. In addition, expensive boxes are required, there are high equip- 3,428,110 Patented Feb. 18, 1969 ment maintenance costs and there is considerable operator fatigue. Therefore, such processes are not completely satisfactory and accordingly not practically used.

The cold box methods generally used can be divided into two types, i.e. those using organic binders and catalysts and those using inorganic binders, such as sodium silicate and gassing techniques. The former type suffers from the disadvantage that it is necessary that the binder system possess a sufliciently long bench or shelf life, i.e. the time interval between the time at which the mix is prepared and the time at which the mix has become so agglomerated that it can no longer be readily and effectively fed into the core box. This results in a long setting or curing time, of the order of thirty minutes or an hour or longer.

The latter type of cold-setting process has a number of disadvantages also, i.e. lack of breakdown (i.e. decomposition of binder and collapse of bonded sand) after casting, excessively high hot strengths, and comparatively low cold strengths, these being associated with the use of inorganic binders. Gassing techniques have a number of advantages and it has been proposed in, for example, US. Patent 3,179,990 to provide a gassing method of producing a core in which the aggregate is mixed with an organic binder. The particular type of binder disclosed in that specification is, however, quite complex and not one of the normal types of binders normally employed in the foundry industry.

The types of organic binders at present in use include natural drying oils, such as oiticica oil, synthetic drying oils based upon polyhydric alcohol esters of ethylenically unsaturated fatty acids, oil-modified alkyl resins, petroleum polymers containing cyclopentadiene or dicyclopentadiene, polyester resins, epoxy resins, urea-formaldehyde resins, melamine-formaldehyde resins and phenolformaldehyde resins. Furfuryl alcohol-based resins and furfuryl alcohol-modified resins are also commonly used.

It has also been proposed for example, as in French patent specification No. 1,132,248, to provide a binder system which includes a drying oil, such as oiticica oil, and a polyisocyanate which increases the rate at which the core develops sufficient stripping strength.

Such a system is also disclosed in United States Patent 3,255,500 wherein a polyisocyanate is employed in a binder comprising a drying oil, modified alkyd resin etc. While such processes have increased the curing rate of the curable binder it has still been the desire of the industry to provide a more rapid curing system which will overcome the inherent deficiencies and disadvantages of the previously employed processes.

Accordingly, it is a principal object of the present invention to provide a method of producing foundry cores and molds which eliminates the inherent deficiencies and disadvantages of prior art processes.

A further object of the present invention comprises an improved method of producing foundry cores and molds wherein a polyisocyanate is employed in conjunction with a curable binder, a rapid curing being attained by the employment of a volatile amine.

A still further object of the present invention comprises an improved method of producing foundry cores and molds having the advantage of a rapid cure of the binder employed by a former mixture of sand, a curable binder and a polyisocyanate, and after introducing such mixture into a core box, mold or pattern, passing a volatile amine therethrough.

Still further objects and advantages of the process of the present invention will become more apparent from the following more detailed description thereof.

The above objects and advantages of the process of the present are achieved by providing a process for the pro duction of foundry cores and molds which comprises preparing a mixture of a foundry aggregate e.g., sand, a conventional curable binder, and a polyisocyanate; introducing such mixture of sand into a core box, mold or pattern to form a green core or mold; and effecting a rapid cure not heretofore attainable by passing a volatile amine into the core box in contact and through the green core or mold.

As the curable binder employed in accordance with the present invention, any of the conventional employed organic binder systems can be employed. Thus, for example, the organic curable binder can comprise an epoxy resin binder, a polyester resin, a petroleum polymer and oil-modified alkyd resin, or a phenol-formaldehyde thermosetting resole resin among others. Again, it is within the scope of the present invention to employ any conventional organic binders capable of being cured from the green core or mold stage by passing therethrough a volatile amine in accordance with the present invention.

Generally, the organic curable binder employed in accordance with the present invention is mixed with sand or other suitable foundry aggregate in the absence of water and, in the presence of an organic solvent. Such an organic solvent can comprise for example, an aromatic solvent e.g., xylene or a conventional halogenated hydrocarbon. The binder is generally employed in the mixture in an amount by weight of about 0.25% to about by weight based on the sand, preferably between about 1% and about 3% by weight.

The organic polyisocyanate employed in accordance with the present invention also can comprise those conventionally employed in the art. Thus, for example, the organic polyisocyanate can comprise an organic di-isocyanate e.g., 4,4-diphenylmethane di-isocyanate or 2,4- or 2,6- toluene di-isocyanate or a higher polyisocyanate e.g., tri-phenylmethane tri-isocyanate. Preferably, the amount by weight of the polyisocyanate employed in sand-curable binder-polyisocyanate mixture is within the range of about 0.25% to about 5% by weight based on the weight of the sand, most preferably the amount of polyisocyanate being within the range of about 1% to about 3 by weight.

Generally, the bench or shelf life of such a foundry mix prepared by admixing foundry aggregates e.g., sand, organic curable binder, and polyisocyanate, e.g., the coated sand, will depend on the particular choice of binder, polyisocyanate and, in addition, solvent utilized; thus, on the average such bench or shelf life will generally be in excess of two hours.

The volatile amine employed in accordance with the present invention to provide the rapid cure of the binder in a manner not before attainable can be any volatile amine capable of effecting the cure of the curable binder. Generally, the volatile amine comprises a primary, secondary or tertiary lower alkyl substituted amine. Thus, for example, suitable volatile amines employed in accordance with the present invention include ethylamine, di-ethylamine, tri-ethylamine, tertiary butyl amine, isopropyl amine, etc.

The process of producing the green core or mold in accordance with the present invention is essentially the same as conventional processes for producing such foundry molds. Thus, in a practical embodiment of the present invention, sand is fed into a combined mixer and conveyor unit as described in US. specification No. 3,268,214, a solution of an epoxy resin in xylene being for example, introduced in a continuous stream into the combined mixer and conveyor unit. A solution of 4,4-diphenylmethane di-isocyanate, for example, in an organic solvent is also introduced into the combined mixer and conveyor unit at a continuous rate. The amount by Weight of resin introduced is, for example, about 1% of the weight of the sand (A.F.S. fineness No. 55), and the amount by weight of the isocyanate is also about 1% of the weight of the sand.

The resin/isocyanate/sand mixture is introduced into a core box, there being a plurality of core boxes moving along the conveyor line, the core boxes being filled successively with the mixture as they are moved past the discharge outlet of the combined mixer and conveyor unit. After being filled with this mixture the core boxes are passed to a station at which each is connected to an air line containing a compressor and, if necessary, suction can be applied to reduce the pressure within the core box. The compressor is then operated to blow a stream of air through a vessel containing a volatile amine, such as triethylamine, the air stream containing the amine vapor then passing into the core box. The length of time for which the stream of air containing the amine vapor is passed into the core box will depend upon the size of the core, the air pressure used and the chemical characteristics of the binder system. The time for which the amine is introduced may be, for small cores, of the order of ten seconds. This compares quite favorably with the rates obtained in hot box processes. After the amine has been blown into the core box for the required length of time, air not carrying the volatile amine is blown into the core box whereby any amine which has not reacted with the polyisocyanate in the mixture will be removed, such blowing operation lasting, for example, for ten to twenty seconds. It has been found that, with any particular binder system, there is a minimum gassing time and that, if the amine is blown into the core box for less than this time, parts of the core box are not cured. If the amine is blown into core box for longer than this minimum gassing time, hardly any increase in strength is obtained.

After blowing the amine into the core boxes, such core boxes are passed successively to a stripping station at which the core boxes are opened and the cores removed. Alternatively, the resin/isocyanate/sand mixture may be blown into a core box and the subsequent gassing and stripping operations carried out immediately at the core blowing machine, such a practice allowing rapid repetitive production of cores.

The time taken to fill each core box with the sand/ resin/isocyanate mixture is of the order of a fraction of a second when a core blowing machine is used and the time that each core box spends at the station at which it is connected in the air line system may be of the order of one minute. It can therefore be seen that the time taken to produce each core is very small.

After curing each core can be subjected to a conventional core-washing process, the wash comprising for example, an aqueous or alcoholic suspension of graphite or zircon.

Although the invention has been described above in relation to the mixing of the sand, resin and isocyanate in a continuous mixer and conveyor, such mixing operation can alternatively be carried out in a batch mixer.

The present invention will now be illustrated by the following specific examples. It is to be understood however, that such examples are presented for purposes of illustration only and the present invention is in no way to be deemed as limited thereto.

Example 1 45 grams of an epoxy resin (No. 828 supplied by The Shell Chemical Company Limited) were mixed with 4,530 grams of sand (Chelford W.S. sand of A.F.S. fineness No. 45), and 45 grams of 4,4-di-phenyl methane di-risocyanate were added and the whole mixed for a period of two minutes. Compression test pieces were rammed using a standard Dietert sand rammer and a mixture of triethylamine vapor and air passed through the test piece while still in the specimen tube. The minimum gassing time required was two minutes and the compression strength obtained was 177 pounds per square inch.

Example 2 The same procedure was carried out as in Example 1 except that 45 grams of a low viscosity, high reactivity polyester resin were substituted for the epoxy resin, the polyester resin being No. 4128 supplied by British Industrial Plastics Limited. The minimum gassing time was 110 seconds and the subsequent compression strength was 175 pounds per square inch.

Example 3 The same procedure was carried out as in Example 1 except that 45 grams of a petroleum polymer consisting mainly of polymerized cyclopentadiene were substituted for the epoxy resin. The minimum gassing time was 140 seconds and the compression strength was 85 pounds per square inch.

Example 4 The same procedure was carried out as in Example 1 except that 45 grams of an alkyd resin manufactured by British Resin Products Limited were substituted for the epoxy resin. The minimum gassing time was 90 seconds and the compression strength obtained was 220 pounds per square inch.

Example 5 The same procedure was carried out as in Example 1 except that 45 grams of a thermosetting resole phenolformaldehyde resin were substituted for the epoxy resin, the resole resin being No. FPR 55 supplied by Fordath Limited. The minimum gassing time was seconds and the compression strength obtained was 150 pounds per square inch.

The mixes described in Examples 1 to 4 had a bench life in excess of one hour whereas the mix of Example 5 had a bench life of the order of ten minutes.

In addition to the employment of the sand or similar foundry aggregates, curable organic binder and polyisocyanate, the mixtures of the present invention can contain conventional additives generally employed in sandbinder systems. Thus, for example, conventional sandbinder systems generally contain catalysts such as cobalt naphthenate, sodium perborate and dibutyl tin dilaurate, such catalysts generally being added to accelerate the cure of the binder system. In accordance with the present invention such conventional catalysts can also be included in the binder-isocyanate-s'and mixtures employed in accordance with the process of the present invention to reduce the minimum gassing time required for the core to obtain optimum stripping strength.

While the present invention has been described primarily with respect to the foregoing specific examples, it is to be understood that the present invention is in no way to be deemed as limited thereto but should be construed as broadly as all or any equivalents thereof.

What we claim then is:

1. A process for the production of foundry moldsand cores which comprises mixing a foundry aggregate with a curable binder and a polyisocyanate, introducing the mixture into a core box, mold or pattern to form a green core or mold and passing an amine into the core box, mold or pattern to eifect curing of the core or mold.

2. A process according to claim 1 in which the curable binder is an epoxy resin.

3. A process according to claim 1 in which the curable binder is a polyester resin.

4. A process according to claim =1 in which the curable binder is petroleum polymer.

5. A process according to claim 1 in which the curable binder is an alkyd resin.

6. A process according to claim 1 in which the curable binder is a phenol-formaldehyde thermosetting resole resin.

7. A process according to claim 1 in which the amine is passed into the core box, mold pattern in the form of a vapor entrained in a gaseous carrier.

8. A process according to claim 7 in which the amine is triethylamine.

9. A process according to claim 1 in which the polyisocyanate is 4,4di-phenylmethane di-isocyanaate.

References Cited UNITED STATES PATENTS 2,913,787 11/1959 Cooper 10638.6

3,032,426 5/ 19 62 Lee 16442 X 3,179,990 4/1965 Freeman 16443 3,255,500 6/1966 En gel et 'al. 16443 FOREIGN PATENTS 1,'l32,248 3/1957 France.

I. SPENCER OVERHOLSER, Primary Examiner.

E. MAR, Assistant Examiner.

US. Cl. X.R.

Disclaimer 3,428,110.J0/mes Walker, Lower Gornal, near Dudley, and Geofi'rey W. Westwood, Walsall, England. PROCESS FOR THE PRODUCTION OF FOUNDRY CORES AND MOLDS. Patent dated Feb. 18 1969.

Disclaimer filed Apr. 24, 1969, by the assignee, Foseco-Fo'rdath 14.6 Hereby enters this disclaimer to claims 1, 6, 7, 8 and 9 of said patent. [Oficial Gazette September 23, 1.969.] 

